Interline read-out system for storage tube



3, 1965 a. w. MANLEY 3,198,977

INTERLINE READ-OUT SYSTEM FOR STORAGE TUBE Filed Aug. 29, 1962 3Sheets-Sheet 1 r +l0v +5v 6o 63\E 52 Sv(W) INVENTOR.

BRIAN WILLIAM MANLEY AGENT 1965 a. w. MANLEY 3,198,977

INTERLINE READ-OUT SYSTEM FOR STORAGE TUBE Filed Aug. 29, 1962. 3Sheets-Sheet 2 8=Max F IGA W(i) IHHEEIEBI Wm lulllflul R(i,i+1) lummulR(i.i 1) IIIIMEIIIHI WW IIEEIIHBI W(i+1 lumuul FIG.5 V FIG.7A

Wm IHIIEIIHEII wm IIIIIBIII R(i,i+1) annual R(i.i+1) IIIIIIIBIIDI WWlifilfluifill w i+1 Infill! FIQSB FIG.7B

INVENTOR BRIAN w. MANLEY AGENT Aug. 3, 1965 a. w. MANLEY INTERLINEREAD-OUT SYSTEM FOR STORAGE TUBE Filed Aug. 29. 1962 3 Sheets-Sheet 3INVENTOR BRIAN W1 MANLEY BY Mi.

AGENT Unite This invention relates to circuit arrangements employingcharge storage tubes, and more particularly to arrangements capable ofproviding non-destructive readout of the information stored.

In general, non-destructive reading in storage tubes is achieved by theuse of a beam modulation process effected with the aid of a gridelectrode interposed between the gun and the target. The insulatingstorage layer is supported on this metallic mesh. Potential variationson the surface of the insulator, established in the writing process,control the passage of an electron beam through the mesh, but thepotentials are such that no electrons from the beam can land on theinsulator. In the case of a display storage tube such as the RCA 7183,the beam strikes a fluorescent screen after passing through the mesh. Inan information storage tube such as the Raytheon CK 7702, the focusedbeam scans over the mesh area, and the transmitted electrons reach afinal electrode to constitute the output signal. Grid modulation readingof this type preserves the stored charge and, in the case of a tube likethe Hughes Memotron operated in a bistable mode, augments it, so thatreading may proceed for long periods.

All the tubes referred to above are very expensive and have had to bedesigned and made specially for non-destructive read-out purposes. Inaddition it is diliicult with tubes of this type (which operate withelectron beams having energies of some 2 or 3 K.V.) to erase the storedinformation completely in a short time. It is a principal object of theinvention to provide improved circuit arrangements adapted for use withcheaper generalpuipose charge storage tubes and having the additionaladvantage that rapid and complete erasure is possible.

In the following, the term secondary-ernissive is intended to coversubstances having a secondary-emission coefficient 6 which is less thanunity when bombarded with electrons of energy less than a certaincritical value known as the first cross-over potential V and asecondary-emission coefficient greater than unity for a range ofbombarding energies above this critical energy. The materials used mayhave a second cross-over potential V where the coefficient is unityagain before becoming Smaller, but materials can also be used in whichsaid second cross-over does not exist for practical purposes or cannotbe determined.

The invention provides a circuit arrangement comprising a charge storagetube having an electron gun for producing an electron beam which gunincludes a cathode, a control grid and an anode, and having a targethaving an uninterrupted secondary-emissive insulating storage surfacewhich target is provided with a backing of elec trically conductivematerial and a collector for collecting electrons obtained by secondaryemission from said target, the circuit arrangement also comprising meansfor causing the beam to eiiect a writing scan in a focused conditionover a first path on the storage surface, means for applying inputsignals between grid and cathode during said scan so as to modulate theintensity of the beam current and cause a charge pattern to be set up onthe storage surface in accordance with the input signals, means forcausing the beam to effect a reading scan over grates atent a secondpath spaced from but adjacent to said first path while maintaining thebeam in a constant-current focused condition and ensuring that thepotential difference between each part of the target scanned and thecathode is greater than the first cross-over potential of the materialof the target but less than the second cross-over potential, it any, andan output circuit for deriving an output signal, simultaneously withsaid reading scan, from the transfer of secondary electrons from thetarget to the collector.

In the above definition the term insulating includes target materialswhich can, in known manner, be rendered conductive by bombardment withhigh-energy electrons but remain non-conductive under bombardment bylow-energy electrons such as are used in the reading process. If thelatter type of material is used, then the writing can be done bybombardment-induced conductivity obtained by scanning with high energy(erg. 6 K.V.) electrons; then the target backing may be maintained at apositive potential with respect to the bombarded surface so that, whenthe writing process renders the target conductive, the potentialgradient through the thickness of the target will transfer positivecharge to the target surface (alternatively a negative backing can causetransier of negative charge).

Alternatively, the writing can be done by secondary emission in whichcase a positive charge pattern is set up and it is necessary to ensurethat the potential difference between each part of the target scannedand the cathode is at all points greater than the first cross-overpotential of the material of the target but less than any secondcross-over potential.

The reading operation employs secondary-emission and is based on theprinciple that the grid action of the target for each reading scan iscontrolled by the electrostatic fields of the written charge pattern ina manner which will be described more fully, and this control effectWill, for convenience, be referred to as co-planar grid action from theanalogy of a control grid lying in the same plane as the target surface(but this term does not imply that the target surface must in all casesbe a plane surface). Said charge pattern is not destroyed by a readingscan because the latter takes place on different parts of the target. Inthe operation of such a circuit arrangement, the output informationobtained during the scan of any element of the second path correspondsto the positive charge which can be deposited on that element despitethe presence of written charge on the adjacent element of the first pathand this in turn :will depend on the amplitude of the input signal andthe magnitude of the negative charge (if any) already deposited on thatpath. (The term element refers to an arbitrary elemental area occupiedby one bit of information in a binary system or one picture element in atelevision or like systom, and should not be taken as implying anyphysical subdivision or discontinuity in the target surface.) In otherwords, the areas unscanned during the reading operation (including theelements of the written charge pattern) take the place of the interposedstorage grid of the aforesaid RCA and Raythcontubes and act so as tocontrol the action of the beam during its reading scan. This controlaction "is quite different since it is no longer a question ofcontrolling the passage of the beam (through grid apertures) to a moredistant, and separate, target. With the present invention What iscontrolled is the ability of secondary electrons to escape to thecollector electrode from the target areas scanned in the readingprocess, and it is this latter process (which can be repeated) thatprovides the read-out signal.

In view of this mode of operation, a charge storage grid is no longerrequired, and this is one of the reasons why a cheaper and simpler tubecan be used for nondestructive reading. One such tube is the MullardTenicon tube (type ME 1260) which is a low voltage, single guninformation storage tube employing a capacitive discharge read-outprocess, and the feature of low voltage operation is a furtheradvantage. This tube has a collector mesh parallel to the target, butthis meshis made entirely of metal and is not a storage mesh; inprinciple, even this mesh can be dispensed with for the purposes of thepresent invention provided that it is replaced by some other form ofcollector electrode (e.g. a peripheral ring) which is suitable in thesense of being able to provide a substantially uniform collecting fieldall over the target.

Arrangements in accordance with the invention may take various forms.For example, the information to be stored may be written on a line or ona raster of parallel lines as half-tone or as bipotential modulation. Inthe case of a written raster, the reading scan is performed between thewritten lines on an interlaced raster; if the lines of the readingraster are substantially equally spaced between lines of the writtenraster (as will be the case in the first embodiment described below),then the output signal for any given element will correspond to a meanof the information stored in the adjacent elements of the twoneighbouring written lines; alternatively, it is possible to place eachreading line much nearer to one of the two neighbouring lines of thewritten raster so that the influence of that line is predominant andcontrols substantially entirely the read-out of information.

As for the process involved in reading out, there is more than one wayin which it can be rendered repetitive. In the first embodimentdescribed below each reading scan is followed by a re-stabilizing scanwhich destroys the pattern generated by the reading process and leads toa half-tone type of operation. For this re-stabilizing scan it isensured that the potential difference between each target elementscanned and the cathode is less than the first cross-over potential ofthe material of the target; although described as a raster scan with adefocused beam, re-stabilization could, with an appropriate tube, beperformed by flooding the whole target since this would not destroy thestored pattern (the reason for this is the same as that given later forthe defocused beam scan described).

In the second embodiment, the re-stabilizing process is omitted and thisleads to a bipotential type of operation. The first reading scan orscans establish the read path at equilibrium potentials determined bythe co-p lanar grid action of the adjacent written elements. Successivereading scans (which may be several thousands) provide signals ofsubstantially constant amplitude generated (as far as can be ascertainedby redistribution eifects in the areas of the reading lines.

The aforesaid two embodiments of the invention will now be described byway of example with reference to the accompanying diagrammatic drawingsas applied to the secondary-emission mode of writing. In the drawmgs:

, FIGURES 1 to 4 illustrate the co-planar grid action;

FIGURES 5A and 5B are charge writing and reading diagrams relating tothe first embodiment,

FIGURE 6 is a circuit diagram related to the first embodiment;

FIGURES 7A and 7B are charge writing and reading diagrams relating tothe second embodiment.

The two embodiments will be described as applied to the Tenicon or to asimilar storage tube having a collector mesh parallel to the targetsurface and magnetic focusing. The co-planar grid action and theoperation of the embodiments will be described, for convenience, withthe aid of specific voltage values; although these are realistic, theyshould not be taken as being limitative in any way.

Referring to the drawings, the so-called co-planar grid action will bedescribed first with reference to FIGURES 1 to 4. This description willbe directed more especially to a half-tone application such as that ofthe first embodiment, but will hold good also for the simpler case of dbipotential operation e.g. as used for the storage of binary informationin the second embodiment.

In each of FIGURES 1 to 3, it should be assumed that an element 61 of aninsulating target is being read by bombardment with an electron beam 62of finite crosssection, while the adjacent element 63 of the target 60on either side of the element 61 have a charge which has been writteninto them in the writing scan. Each of these figures is, in effect,'thecross-section of two written lines 63 and an interleaved read line 61.

The more negative the adjacent written potentials, the fewer secondaryelectrons can escape from the target 60 to a collector mesh 64 arrangedin front of the target and parallel thereto.

FIGURE 1 represents the case when all elements of the target 60 are atthe same potential, in which case all secondary electrons produced atthe bombarded element 61 are drawn to the collector mesh 64, thetrajectories of these secondary electrons being indicated by 65.

In FIGURE 2 the negative potential regions 63 surrounding the bombardedelement 61 influence the potentials in front of the target 60 in the wayindicated by the lines representing equipotential surfaces. In thiscondition only a portion of the secondary electrons liberated at thebombarded element and indicated by 65 escape to the collector 64, someof the secondary electrons being returned to adjoining regions of thetarget as indicated by trajectories 66.

In FIGURE 3 the potential of the surrounding negative target areas 63with respect to the potential of element 61 is reduced to a value (e.g.10 volts) at which value the number of secondary electrons escaping tothe collector (trajectories 65) is assumed to be equal to the number ofprimary electrons (beam 62) striking the target area. This can bereferred to as the equilibrium condition, and it can be said that inthis condition the value (e.g. 10 v.) of the negative potential of thesurrounding areas is the value at which the effective (apparent)secondary emission coelficient of the bombarded element 61 is reduced tounity. Although an equilibrium voltage of -l0 v. is a realistic example,the actual potential difference at which the effectivesecondary-emission coeificient is reduced to unity will be determined bythe width of the space between the written lines and the magnitude ofthe electric field normal to the bombarded element, resulting from thepotential of the collector mesh 64.

When an element of the target surface is bombarded in this condition,the potential of the element does not change. What this equilibriumpotential is will depend upon the spacing between the Written lines. Forthe line spacings used in the following embodiments the equilibriumpotential is about l0 v. Thus on a target element in the reading pathwhich has adjacent written elements at this potential, no net charge isdeposited and no signal current flows in the signal circuit. This signalcircuit may be incorporated either in the connection to a conductivebacking plate of the target 60 or in the connection to the collectormesh 64.

FIG. 4 represents the changes in the aforesaid effective secondaryemission coefiicient 5 of an element 61 in dependence upon changes inthe negative potential of the adjacent written elements 63 of FIGURES 1to 3. Thus. a coefficient of unity (equilibrium) corresponds to anadjacent potential of 10 v. (FIGURE 3) while the maximum coefficient(which may be about 4) corresponds to an adjacent potential of zerovolts (FIGfl).

The first embodiment will now be described with reference to FIGURES 5A,5B and FIGURE 6 on the assumption (discussed later) that the informationon one Written line is very similar to the information on the nextwritten line.

In FIGURE 5A, like the FIGURES 53, 7A and 7B schematically representingthe charge distribution on a given part of the surface of the target W(i) and W(i+l) represent two adjacent written charge lines of a writingscanof rectangular raster form while RU, i+1) represents a line of asimilar reading scan and the potential pattern left thereby. In FIGURE5B, R(i, i-l-l) represents the uniform charge set up by a re-stabilizingscan on read line R(i, i-l-l). For convenience of description, the scanlines are divided arbitrarily into discrete elements difiering by one ormore volts, but in practice the Writing is efiected in a continuoushalf-tone manner corresponding e.g. to a video signal (the presence ofintermediate 5- volt elements represents the half-tone nature of theoperation). I

Let it be supposed that, in the operation of writing, the target isscanned so as to produce a series of separated parallel lines, such asW(i) and W(i, i+1), of modulated charge in which the target potentialvaries from an initial value of about 10 volts, positively to a maximumof zero volts representing peak stored signal amplitude. Let it besupposed, also, that the spaces between the lines are uniformly at v.The operation of reading comprises a readin scan and also (in thisexample) a sub sequent restabilizing scan.

With the cathode potential at about l00 v., the focused beam is scannedalong the spaces between the written lines. This is like the interlacedfield of a television scan. When the beam strikes the target surface,secondary electrons will be emitted. Whether or not these secondaryelectrons escaped to the collector mesh, which is held at about +200 v.and is spaced e.g. a few millimetres from the target, will depend uponthe target potential in the surrounding target areas, in particular theadjoining elements in the Written lines W on either side of the readline R.

If the element read lies between two written elements which are at zerovolts, all secondaries escape to the collector (in the manner of FIGURE1), the potential of the read element shifts positively by a few volts(to +2 v. in FIGURE 5A), and the maximum signal current lows. Fromelements having intermediate potentials in the ad- 'acent parts of thewritten lines (e.g. -5 v. in FIGURE 5A) a smaller proportion of thesecondary electrons reach the collector thus leaving the target elementat a lower positive potential (e.g. +1 V. in FIGURE 5A) and producing asmaller signal current (this corresponds to FIGURE 2). If the elementread lies between written elements at volts (FIGURE SA), the previouslydescribed equilibrium condition applies (see FIGURE 3) and the readingbeam fails to alter the initial potential of zero volts and no signalcurrent is observed. In the read scan along a line R, therefore, thesignal current ampli tude will be directly related to the potentialpattern established on the target along the lines \V during the writingoperation. The positive potentials (0 to +2) now estab lished in thetrack R(i, i+1) of the reading scan are similarly related to thepotentials of the adjoining areas established during writing.

In preparation for a further reading scan, the re-stabilizing scan nowrestores to zero the potentials of the elements scanned during reading.lrVith the cathode at zero potential the beam is scanned over the sametrack R as that followed during reading. Areas which had been shiftedpositively during reading Will be cathodepotential stabilized to zero(FIGURE 53) and, at the completion of this scan, the target is ready fora further reading scan. The landing current during this scan issubstantially equal in amplitude and of reverse polarity to thatobtaining during the reading scan, and this signal could be used as anoutput in addition to that obtained in the reading scan. However, it hasbeen found that the resolution of the restabilizing signal was not asgood as that obtained in the reading scan, due to the beam spot size inthis scan adding a further degradation to the stored information. Forthis reason, in this embodiment, only the first signal is used and adefocused beam is used in the restabilizing scan to avoid the problem ofscan registration.

The difficulty of scan registration arises as a result of the differentpotentials applied to the tube in the reading and re-stabilizing scansThis results in a slight rotation of one scan with respect to the other(When magnetic focusing is used) so that the re-stabilizing scan pathsdo not coincide with the reading paths.

Since the target potentials in the written paths are nowhere greato thanthe cathode potential in the restabilizing scan, no electrons can landon the written area in this scan. It is therefore not necessary that inrestabilizing the scan paths be identical with those in the reading scanprovided they cover these paths completely. This is most easilyaccomplished by defocusing the beam during the re-stabilizing scan sothat the beam current falls uniformly on the reading paths.

The output signal amplitude decays in time, with successive read-outs,in a Way similar to that observed in half-tone display storage tubes. Indisplay storage tubes, the decay is the result of positive ions,generated by the reading beam, falling on the storage insulator andcharging it positively. In the Tenicon (when used in the presentarrangement), written areas of the target are similarly shiftedpositively, in this case by the release of secondary electrons generatedby the arrival of primary electrons in the skirts of the electron beamused in the reading process, which skirts overlap the written paths. Inboth cases this positive shift of the target surface potential resultsin the DC. level of the signal from all parts of the target increasingin time towards the value corresponding to the collection of allsecondaries generated by the reading beam in the reading paths at thecollector electrode. Finally a uniform white signal is obtained from allparts of the target.

The useful storage time depends upon the spacing of the write and readscan lines but, with 200 read scan lines in a target diameter of 25 mm.as is available in the Tenicon, about 1000 read-outs may be obtainedbefore the amplitude of the output signals has dropped by a factor of 2.The resolution in the line scan direction under these conditionscorresponds to about 400 picture elements for a target diameter of 25mm.

Erasing consists of restoring all the target areas which are to be usedfor Writing to the most negative value representing zero stored signal(-10 v.). This is accomplished by scanni g the target with the cathodeat a potential of l() v. and the collector at a potential such that'thepotential difference between collector and cathode during this scan isless than the first cross-over potential. All scanned areas of the targeW'll then be stabilized at -l() v. A defocused beam may be used to avoidscan registration problems since it is of no consequence that the areasof the target which are to be used for reading are also stabilized at-l0 v. by this operation: in fact, the read elements will be drivenpositively in the first few reading scans and will thus adopt theirequilibrium values.

A notable feature of the erasing process is that it leaves no residualimage. The problem of a residual image after erasing is common to manynon-destructive readout tubes and is probably a function or" the highvoltage at which the target is bombarded.

One possible circuit arrangement suitable for the first embodiment isshown schematically in FIGURE 6. The tube, of which the envelope is notshown, is a Mullard Tenicon tube W h anodes A2 and A3 connectedtogether. The output circuit employs a conductive backing "iii of theinsulating target 6% as a si nal plate connected to an output load "illand coupled to an amplifier. The first anode Al has applied to it over acapacitor 79 negativegoing tlyback suppression pulses 78, which, ofcourse, have to be related to the particular scans used for a ing andreading (the means for ensuring this correlation are omitted forsimplicity).

The voltage values given are, again, realistic (for the Tenicon) but notlimitative. Magnetic deflection and focusing by means of coils 72 and 73respectively are employed in the conventional manner.

Separate time-base systems (TB1, TBZ) for feeding the coil 72 are shownfor writing and reading: for simplicity, the line and field circuits andthe line and field deflection coils are shown and treated together. Aswill be appreciated, the write and read scans may be effected (by TB1and T132) at different speeds so long as the requirement of correctraster interlacing is maintained. By means of switch 74 either TB1 orT32 is operatively connected With coil 72 The input signal 75 is appliedto the beam-intensity control electrode of grid G, and the grid-cathode(G-K) conditions for the various operations are determined by a twinganged switch system Sa-Sb having four positions (1 to 4).

The beam is infocus when the focus coil is energized, 200 volts areapplied to anodes A2A3, and the cathode K is at l v. (This means thatthe beam is defocused for switch positions S1 and S4 where the cathodeis held at v. and earth respectively.)

The system provides for the following operations:

(1) Erasure.-This is effected by one or more scans in which the target60 is stabilized all over at 10 v. The switches Sa-Sb are in position 1so that grid G and cathode K are at the same potential which allows asufficiently large beam current to effect erasure. Preferably time-baseTB1 is used (i.e. the same as for writing) but TB2 can also be usedsince the beam is defocused.

(2) Writing.-Switches Sa-Sb are in position 2 and time-base TB1 isoperatively connected to the deflection coil 72. The DC. bias on thegrid G is -60 v. with reference to cathode K which is the cut-offpotential of the gun. The input signal 75 (applied through Sb) has apeak-topeak amplitude of 10 volts from 0 v. to +10 v.) and this (withthe available beam current) is arranged to drive the target from 10 v.to 0 v. at points of peak signal amplitude. During the writing process asignal current flows through the output circuit connected to plate 70and it may be desirable, in special cases, to monitor or utilize thissignal.

(3) Reading-Switches Sa-Sb are in position 3 and by means of switch 74now time-base TB2 is switched into circuit so as to displace the scansto an interlaced position between the written lines. The grid is at -50v. with reference to cathode and this determines the amplitude of theoutput signal across load 71.

(4) Re-stabilization.Switches Sa$b are in position 4 p and time-base T32is kept in circuit. The grid voltage is zero with reference to thecathode and this permits large beam currents, but the target will onlyaccept the amounts of current needed for re-stabilization.

The second embodiment will now be described with reference to FIGURES7A, 7B. In FIGURES 7A-7B, two successive written lines are shown at W(i)and W(z'+1). The information is of a binary character and is stored in abipotential manner so that each element assumes one of two voltages. Theinformation is the same on both of the written lines and this will bedis cussed later. A circuit generally similar to that of FIGURE 6 can beused as far as writing and reading are concerned, with switches inpositions 2 and 3 respectively.

In FIGURE 7A, R(i, i+1) represents the charges set up by a reading scanon read line (i, i+1) immediately after the beam has passed. In FIGURE7B, R(i, i+1) represents the same charges as modified after a lapse oftime corresponding to about one field scan of the read raster but beforethe beam has again scanned the elements shown. This modified pattern isready for a further read scan during which it will be restored to thevalues of FIGURE 7A as will be explained.

To adopt, again, specific values for convenience of de scription, whichvalues are suitable for the Tenicon tube, the written information (thatis in the tracks W(i) and W(i+1) is shown as a square Wave potentialmodulation 8 having an amplitude of 10 v. (FIGURE 7A). To read theinformation, the cathode potential is set to about v., and the beamtracks between the written lines W as in the first method. In this case,however, the reading operation can be repeated without an intermediatere-stabilization process.

During the first reading scan (or the first few read cans) all the readelements in the track R are raised positively (from an initial zerovoltage) by secondary emission until the coplanar grid action of thewritten elements prevents further positive excursions (this is theequilibrium state of FIGURE 3 and corresponds, for a practical case, toa difference of 10 v. between a read element and the adjacent writtenelements; it takes the read elements up to +10 v. or -20 v. depending onwhether they lie between 0 v. or +10 v. written elements). In thiscondition the potential pattern on'the target is as shown in FIGURE 7Awhere the most positive parts (+20 v.) of the read line are those whichadjoin the corresponding positive parts (+10 v.) of the written lines.Although, as explained in the case of FIGURE 3, an equilibrium voltageof --10. v. is a realistic example, the actual potential difference atwhich the effective secondary-emission coefficient is reduced to unitywill be determined, again, by the width of the space between the writtenlines W and the magnitude of the electric field normal to the bombardedelement, resulting from the potential of the collector mesh.

During subsequent line scans of the same reading raster, secondaryelectrons are redistributed to some extent but can go only to thoseparts of the target more positive than the element from which they areliberated. They fall therefore only on the most positive elements (+20v.) indicated in FIGURE 7A. Thus the potential of such an elementchanges negatively (e.g., to +19 V.FIGURE 7B) during the course of areading scan until the next arrival of the beam at that element. Duringthe passage of the beam over that element, the area is again chargedpositively to its maximum permissible potential (cg. from +19 to +20v.), 'by a loss of electrons to the collector, thus giving rise to anoutput signal from the target. As for the read elements at +10 v., theirpotential does not drift negatively during the progress of a read scanand therefore substantially no signal output occurs when they arere-scanned (this is quite in order in the case of binary information) aIf the elements which had reached the highest positive potential (+20v.) were to remain at that potential during the course of each readingscan, they would always be found (at the next scan) in the equilibriumcondition where the effective'secondary emission coefficient is unity.If this were so, they could not be shifted in potential by the passageof the beam during the new reading scan and they could therefore notgive an output in any reading scans other than the first, or first few,reading scans. It is only because of the re-distribution effect justdescribed (whereby an element at +20 v. drifts down to, say, +19 v.during the remainder of each reading scan) that more than one read-outis possible without the need for a re-stablilizing scan such as thatused in the first embodiment.

As inthe first method, as successive read scans are performed, electronsin the skirts of the electron beam land in the written areas and chargethem positively by a small fraction of a volt in each scan, due tosecondary emission. In this method, however, this does not aflfect theoutput signal amplitude since the potential of the area between thewritten lines, which is determined by coplanar grid action, will riseslowly in a similar amount. The modulation depth on the target istherefore maintained, while all areas of the target move positively atabout the same slow rate. The storage time is limited by the targetpotential thus approaching that of the collector. As in the half-tonemethod of the first embodiment, the storage time depends upon linedensity but, with 200 read lines in a target diameter of 25 mm., about9000 read-outs may be obtained before the amplitude of the output signaldrops by a factor of 2. It is worth noting that the output signal infact remains sensibly constant during about the first 8000 readoperations and falls rapidly in the last 1000 scans.

The second embodiment may, as aforesaid, employ a circuit similar tothat of FIGURE 6 with the switch positions 2 and 3 employed for thewriting and reading scans respectively as before. The main change in theuse of the circuit is that switch position 4 is used instead of switchposition 1 for the erasing process while the collector is switched from+200 v. to a potential (e.g. +20 v.) lower than the first cross-over(the latter may be about 50 v.). This can be done by a switch ganged toSalb. By this means the entire target surface is stabilized at 0 v.instead of 10 v. in this case the circuitry for position 1 becomesredundant.

Resolution in the line scan direction is similar to that of the firstembodiment but no evidence of half tones has been observed even whenhalf-tone signals are applied as the input; the reason for this is notfully understood and it is for this reason that the example has beendescribed as applied to binary information.

It will be apparent that, in the nature of the embodiment described,there are in practice certain limitations to the type of informationwhich can be stored. in FIGURES 5A-5B the output signal is effectivelyan average value of the adjacent input signals in the two written scanlines lying on either side of the reading line. Such processes may bepermissible for television pictures, and indeed offer a convenient wayof performing linear interpolation for purposes of bandwidthcompression; what makes this possible is the statistical likelihood ofadjoining lines of a television picture being very similar (for patternsin which the information varies rapidly in a direction at right anglesto the line scan this method may not be suitable). Such interpolation ispossible because, when there is a difference between two adjacentwritten lines, the output is, as aforesaid, a mean (not necessarily thearithmetical mean) between the two written signals.

in the case of FIGURES 7A, 7B the output would also be a mean betweenthe two adjacent written lines W but this does not have muchsignificance in the case of binary information, particularly when (asshown) the two 21 jacent lines contain identical information. Inpractice, this may be achieved by effecting the Writing operation with aspot-wobbled beam so that the written information is concentrated on twospaced parallel lines between which the reading will take place.

'Where interpolation is not wanted, the problem of differing informationon adjacent writing lines can be solved as aforesaid, by arranging thateach reading line R is kept much closer to one Written line than to theother. This modification can be applied to FIGURES 5A, 5B and also toFIGURES 7A, 7B. The output is still influenced by two written lines butthe effect of one written line is greatly preponderant. As a furthersolution, storage may be limited to a single line in cases where araster is not required.

In the two embodiments described, the collector mesh may convenientlyhave a transparency of about 60 to 70% with 750 meshes per inch, and itmay be disposed at a distance from the target within the range of 2 tomm., preferably a distance of 3 mm. The material of the target may bemica.

The bi-potential embodiment described has the advantage of being adaptedto maintain the same conditions, both for reading and for writing, atall the circuit elements affecting the focus of the beam.

Whereas the arrangement illustrated has been described as applied tosecondary-emissive writing, the arrangement may be adapted to operatewith bombardment-induced conductivity (8.1.0) writing. For this purposethe target material must be changed from mica to a material having BJLC.properties (eg. ZnS or magnesium fluoride) and the DC. voltage appliedto the end of the output load 71 remote from backing plate '70 may bechanged from earth to a positive value of about 20 volts. In this casethe charge pattern is written as positive charge but it may be changedto a negative charge pattern by a suitable change of target material andchanging the +20 v. (applied to the load), to, say, -20 v.

Although the embodiments described employ the backing of the target as asignal plate from which the output is obtained, it is ossible in somecases to derive the output from the collector.

Instead of using a writing scan on a first path followed by a readingscan on a second path adjacent to the first, the invention may bemodified so that a two-dimensional char e pattern or picture is set upindependently of the location of the intended reading scan, said patternor picture being then, in effect, cut into separate strips by theinitial action of the line scans of a reading scan of raster form; afterthis initial action the reading scan will have formed for itself targetareas which can then act as the second paths along which subsequent readscans can effect read-out by co-planar grid action. In all thesemodified arrangements the operation is such that the initial readingscans establish on the target a reading path or paths within the area ofthe originally established charge pattern. The reading process thentaking place along these paths will sense the content of the storedcharge pattern in the areas adjacent to the paths, in the mannerpreviously described.

The first of these modifications provides an arrangement wherein themeans for causing the writing scan are adapted to give said scan theform of a raster of lines and wherein the means for causing the readingscan are adapted to give the latter scan the form of a raster of lineswhich intersect the written lines at a two-dimensional array ofintersection points. A. typical application of this arrangement is theconversion of a display from a radial (P.P.l.) radar-type raster to arectangular raster of parallel lines.

A second modification provides an arrangement wherein the charge storagetube is 1-1 camera tube having a secondary-emissive insulating storagesurface, and wherein the means for causing a writing scan and forapplying input signals are replaced by means for effecting opticalprojection of the information to be stored to produce a two-dimensionalpattern or picture on said storage surface while the means for causing areading scan are adapted to give said scan the form of a raster ofparallel lines. In this case the target must have 6 l at a convenientprimary electron energy. In one example the camera tube is of thevidicon type and the arrangement is adapted to set up a charge patternon the target in response to said optical projection by the action ofphotoconduction in the target material. In this case the readingoperation is very similar to that described for a Tenicon or like tube.Although vidicon tubes typically do not have suitable secondaryemissivetargets, it is a simple modification to the manufacturing process torectify this. In particular, the target material (e.g. Sb S can beevaporated in vacuum rather than (as is usual) in an inert gasatmosphere held at a few Hg pressure.

In another example the camera tube is of the image- Orthicon type andthe arrangement is adapted to set up a charge pattern on the target inresponse to said optical projection by the action of photo-emission fromthe target. In an lmage-Orthicon there is no target backing electrode inthe conventional sense, therefore, the tube would be modified to use aninsulating target (eg. MgO) rather than the more usual semi-insulatingglass target.

Vhat is claimed is:

1. An electrical signal stora e system comprising a charge storage tubehaving a target for storing electric charges, said target comprising asecondary-emissive storage surface composed of an insulating materialand an electrically conductive backplate, means for projecting a beam ofelectrons onto said target surface, a collector electrode positionednear said target for collecting secondary emission electrons from saidtarget, beam intensity control means in said tube, means for applying aninput signal to said intensity control means thereby to modulate thebeam current, means for focusing and scanning said modulated beam over afirst path on said target storage surface thereby to write a chargepattern on said surface determined by said input signal, means forreading out.

the charge pattern of information stored in said first path comprisingmeans for maintaining said beam in a constant current focused conditionand means for scanning said beam over a second path on said surface inclose proximity to said first path, said read-out means furthercomprising means for controlling said beam during a read scan so that aneffective velocity of said beam is produced which is greater than thefirst secondary emission cross-over point of said target material and inwhich the secondary emission coefiicient is maintained greater thanunity, and an output circuit responsive to signals produced by said readscan for deriving an output signal determined by the charge patternstored in said first path.

2. An electrical signal storage system comprising a charge storage tubehaving an electron gun for producing an electron beam and a targetlocated in the path of said beam, said gun comprising a cathode, controlgrid and anode, said target comprising a secondary-emissive storagesurface composed of an insulating material and an electricallyconductive backplate, a collector electrode positioned near said targetfor collecting secondary emission electrons'from said target, means forfocusing said electron beam, means for scanning said beam over a firstpath on said target storage surface, means for applying an input signalto said tube grid during said scan thereby to intensity modulate saidbeam current so as to write a charge pattern on said storage surface,read-out means for the charge pattern of information stored in saidfirst path comprising means for maintaining said beam in a constantcurrent focused condition and means for scanning said beam over a secondpath on said surface which is spaced from but adjacent to said firstpath, means for applying an operating voltage between said cathode andtarget during said read scan which is greater than the first cross-overpotential of said target material and in which the secondary emissioncoefficient is maintained greater than unity, and means for providing arestabilizing scan subsequent to the start of a read scan comprisingmeans for scanning said electron beam over said second path and meansfor applying an operating voltage between said cathode and target whichis less than the first cross-over potential of the target material.

3. An electrical signal storage system comprising a charge storage tubehaving an electron gun for producing an electron beam and a targetlocated in the path of said beam, said gun comprising a cathode, controlgrid and anode, said target comprising a secondary emissive storagesurface composed of an insulating material and an electricallyconductive backplate, a collector electrode positioned near said targetfor collecting secondary emis sion electrons from said target, means forfocusing said electron beam, means for scanning said beam over saidstorage surface in a first raster of substantially parallel spacedlines, means for applying an input signal to said tube grid during saidscan thereby to intensity modulate said beam current so as to Write acharge pattern on said storage surface, read-out means for the chargepattern cathode and target during said read scan which is greater thanthe first cross-over potential of said target material and in which thesecondary emission coefiicient is maintained greater than unity, and anoutput circuit responsive to signals produced by said read scan forderiving an output signal determined by the charge pattern stored insaid first raster of lines.

4. Apparatus as described in claim 3 wherein each read line of saidsecond raster of lines is located substantially equidistant between twoWrite lines of said first raster of lines and wherein the lines of saidfirst raster are parallel to the lines of said second raster.

5. Apparatus as described in claim 3 wherein the read lines of saidsecond raster are parallel to the write lines of said first raster andwherein each read line is located closer to one of the two adjacentwrite lines than it is to the other of said two adjacent Write lines.

6. An electrical signal storage system comprising a charge storage tubehaving an electron gun for producing an electron beam and a targetlocated in the path of said beam, said gun comprising a cathode, controlgrid and anode, said target comprising a continuous secondaryemissivestorage surface composed of an insulating material and an electricallyconductive backplate, a collector electrode positioned near saidtarget'for collecting secondary emission electrons from said target,means for focusing said electron, beam, means for scanning said beamover a first path on said target storage surface, means for applying aninput signal to said tube grid during said scan thereby to intensitymodulate said beam current so as to write a charge pattern on saidstorage surface, means for applying an operating voltage between saidcathode and target during said Write scan which is greater than thefirst cross-over potential of said target material and less than anysecond cross-over potential thereby producing, a charge pattern on saidtarget by secondary emission of said target electrons, read-out meansfor the charge pattern of information stored in said firstpathcomprising means for maintaining said beam in a constant currentfocused condition and means for scanning said beam over a second path onsaid surface which is spaced flOIll but adjacent to said first path,means for applying an operating voltage between said cathode and targetduring said read scan which is greater than the first cr0ss-overpotential of said target material and less than any second cross-overpotential, and an output circuit for deriving an output signaldetermined bythe charge pattern stored in said first path.

7. An electrical signal storage system comprising a charge storage tubehaving an electron gun for producing an electron beam and a targetlocated in the path of said beam, said gun comprising a cathode, controlgrid and anode, said target comprising a continuous secondaryemissivestorage surface composed of an insulating material and anelectricallyconductive backplate, a collector electrode positionednear'said target for collecting'see ondary emission electrons from saidtarget, means for writing information signals on said target surfacecomprising means for scanning said beam over'a first path on said targetstorage surface, means including said electron gun for focusing saidelectron beam during the write scan and means responsive toan inputsignal for modulating said beam current so as to write a chargepattern'on said storage surface path, means for reading-out the charge apattern stored in said first path comprising means for maintaining aconstant current focused beam and means for scanning said beam over asecond path on said surface spaced from said first path, means forproviding a potential difference betwen said cathode and target which isgreater than the first cross-over potential and less than a secondcross-over potential of said target material, and anoutputcircut'responsive to signals derived from said target during said readscan.

8. Apparatus as described in claim 7 further compris- 13 ing means forrestabilizing the elements comprising said second path to a referencepotential of said cathode, said restabilizing means comprising means forscanning said electron beam over said second path subsequent to a readscan, means for defocusing said electron beam during said restabilizingscan and means for providing a potential difference between said cathodeand target which is less than the first cross-over potential of thetarget material,

9. Apparatus as described in claim '7 comprising means for erasing saidstored charge pattern from the target, said erasing means comprisingmeans for scanning the electron beam over the target surface, means fordefocusing said electron beam, and means for stabilizing said targetsurface at a potential value representative of zero stored signal, saidstabilizing means comprising means for providing a potential differencebetween said cathode and target which is less than the first cross-overpotential of the target material.

it Apparatus as described in claim 7 wherein said scanning meanscomprises deflection means located outside of said electron beam, firstand second deflection circuits and switch means for selectively couplingsaid first and second deflection circuits to said deflection means.

11. Apparatus as described in claim it) further com prising an outputcircuit which comprises an impedance element connected to said targetbackplate, means connecting said target to ground, means for supplying apositive voltage to said collector thereby to attract secondary emissionelectrons of said target, and second switch means for supplyingoperating voltages to said cathode and control grid, said second switchmeans having a first and second position for supplying negative voltagesto said cathode and grid, said grid being biased more negative than saidcathode into the region of beam cut-oil, and said second switch meanshaving a third position connecting said cathode and grid to ground.

12. Apparatus as described in claim 7 wherein said collector comprises agrid-like structure interposed be tween the cathode and target anduniformly spaced from the target surface.

13. Apparatus as described in claim 7 wherein said write scanning meansfurther comprises means for defiecting said beam to produce a firstraster of lines and said read scanning means further comprises means fordeflecting said beam to produce a second raster of lines in directionsdifferent than said first raster whereby the lines of said second rasterintersect the lines of said first raster in a two-dimensional array ofintersection points.

14. A television storage tube system comprising a charge storage tubehaving an electron gun for producing an electron beam and a targetlocated in the path of said beam, said gun comprising a cathode, controlgrid and anode, said target comprising a secondary-emissive storagesurface composed of an insulating material and an electricallyconductive backplate, a collector electrode positioned near said targetfor collecting secondary emission electrons from said target, means forfocusing said electron beam, said target material being responsive to aninput light pattern for storing an information charge pattern thereondetermined by said light pattern, means for reading said stored chargepattern comprising means for maintaining a constant current focused beamand means for scanning said beam over said target to form a raster ofspaced parallel lines, said charge pattern being stored in theinterstices between the lines of said read scan, means for providing apotential difference between said cathode and target which is greaterthan the first cros over potential and less than a second cross-overpotential of said target material, and an output circuit responsive tosignals derived from said target during said read scan.

15. Apparatus as described in claim 14 wherein said camera tube is ofthe vidicon-type and said target storage surface comprises aphoto-conductive material which stores a charge pattern in response to alight pattern by the action of photo-conduction in said target material.

16. Apparatus as described in claim 14 wherein said camera tube is ofthe orthicon-type and said target storage surface comprises aphoto-emissive material which stores a charge pattern in response to alight pattern by the action of photo-emission from said target.

17. The method of retrieving information stored as a charge pattern in afirst path on the surface of a signal storage screen composed of asecondary-emissive insulating material which comprises applying anoperating potential to said storage screen which is greater than thefirst cross-over potential of said screen material, scanning an electronbeam over a second path on said screen surface which is adjacent to saidfirst path while maintaining said operating potential thereby toliberate secondary electrons from said screen, and deriving an electricsignal in response to said liberated electrons and determined by thecharge pattern stored in said first path.

18. The method of retrieving information stored as a charge patternalong a first path on the surface of a signal storage screen composed ofa secondary-emissive insulating material which comprises applying anoperating potential to said storage screen which is greater than thefirst crossover potential of said screen material and of sufficientvalue to maintain the secondary emission coefiicient of said materialgreater than unity, scanning a constant current focused electron beamover a second path on said screen surface which is adjacent to saidfirst path while maintaining said operating potential thereby toliberate secondary electrons from said screen, and subsequently scanningthe electron beam over said second path while maintaining said operatingpotential at a value less than the first cross-over potential of saidscreen material.

19. The method of storing information as a charge pattern on the surfaceof a signal storage screen composed of a secondary emissive insulatingmaterial and retrieving said information which comprises scanning anintensity modulated electron beam over a first path on said storagescreen thereby to store .a charge pattern thereon in accordance with thebeam modulation, scanning a constant current focused beam over a secondpath on said screen surface which is adjacent to said first path whilemaintaining said screen at a voltage which is greater than the firstcross-over potential of said screen material and less than the secondcross-over potential of said screen material thereby to liberatesecondary electrons from said screeii, and subsequently defocusing andscanning the electron beam over said second path while maintaining saidscreen at a voltage less than the first cros -over potential of saidscreen material.

it The method of storing and retrieving information in an electrondischarge storage device having a cathode for producing an electronbeam, a collector electrode for secondary electrons and a target platecomposed of a secondary-emissive insulating material, said methodcomprising scanning a focused intensity modulated electron beam over afirst path on said target surface while maintaining a potentialdifference between said cathode and target which is greater than thefirst cross-over potential of said target material and less than thesecond cross-over potential of said material thereby to liberatesecondary electrons and store a charge pattern thereon in accordancewith the beam modulation, scanning a constant current focused beam overa second path on said target surface which is adjacent to said firstpath while maintaining a potential difference between said cathode andtarget which is greater than the first cross-over potential and lessthan the second cross-over potential of said target material thereby toliberate secondary electrons from said second path, defocusing andscanning the electron beam over said second path while maintaining apotential difference between said cathode and target which is less thanthe first cross-over potential of said target material thereby 16 tostabilize the elements of said second path at cathode f r n es Cited bythe Examiner potential, and when desired, erasing said stored informa-Knoll and Kazan: Storage Tubes John Wiley and tion by defocusing andscanning the electron beam over Sons, Inc, New York, 1952 said firstpath While maintining said cathode at a potential representing zerostored signal and maintaining the poten- 5 D A I R I U H PrimaryExaminer tial difference between said cathode and collector electrodeless than the first cross-over potential. ROBERT SEGAL Examiner PatentNor 5,198,9 7

Brian William Manley It is hereby certified that error a ent requiringcorrection and that the s corrected below.

ppears in the above numbered pataid Letters Patent should read as Column4, lines 31 and 36, for "10'', each occurrence, read -l0 column 8, line8, for "cans" read scans line 15, for "-20" read +20 same column 8, line0, for "in" read by r m Signed and sealed this 19th day of July 1966aSEAL) rttest:

RNEST W. SWIDER EDWARD J. BRENNER ttesting Officer Commissioner ofPatents August 3, 1965

1. AN ELECTRICAL SIGNAL STORAGE SYSTEM COMPRISING A CHARGE STORAGE TUBEHAVING A TARGET FOR STORING ELECTRIC CHARGES, SAID TARGET COMPRISING ASECONDARY-EMISSIVE STORAGE SURFACE COMPOSED OF AN INSULATING MATERIALAND AN ELECTRICALLY CONDUCTIVE BACKPLATE, MEANS FOR PROJECTING A BEAM OFELECTRONS ONTO SAID TARGET SURFACE, A COLLECTOR ELECTRODE POSITIONEDNEAR SAID TARGET FOR COLLECTING SECONDARY EMISSION ELECTRONS FROM SAIDTARGET, BEAM INTENSITY CONTROL MEANS IN SAID TUBE, MEANS FOR APPLYING ANINPUT SIGNAL TO SAID INTENSITY CONTROL MEANS THEREBY TO MODULATE THEBEAM CURRENT, MEANS FOR FOCUSING AND SCANNING SAID MODULATED BEAM OVER AFIRST PATH ON SAID TARGET STORAGE SURFACE THEREBY TO WRITE A CHARGEPATTERN ON SAID SURFACE DETERMINED BY SAID INPUT SIGNAL, MEANS FORREADING OUT THE CHARGE PATTERN OF INFORMATION STORED IN SAID FIRST PATHCOMPRISING MEANS FOR MAINTAINING SAID BEAM IN A CONSTANT CURRENT FOCUSEDCONDITION AND MEANS FOR SCANNING SAID BEAM OVER A SECOND PATH ON SAIDSURFACE IN CLOSE PROXIMITY TO SAID FIRST PATH, SAID READ-OUT MEANSFURTHER COMPRISING MEANS FOR CONTROLLING SAID BEAM DURING A READ SCAN SOTHAT AN EFFECTIVE VELOCITY OF SAID BEAM IS PRODUCED WHICH IS GREATERTHAN THE FIRST SECONDARY EMISSION CROSS-OVER POINT OF SAID TARGETMATERIAL AND IN WHICH THE SECONDARY EMISSION COEFFICIENT IS MAINTAINEDGREATER THAN UNITY, AND AN OUTPUT CIRCUIT RESPONSIVE TO SIGNALS PRODUCEDBY SAID READ SCAN FOR DERIVING AN OUTPUT SIGNAL DETERMINED BY THE CHARGEPATTERN STORED IN SAID FIRST PATH.